A ground-Fault Circuit-Interrupter (hereby referred to as GFCI) is a device that is intended to reduce the chance of accidental electrocution by interrupting the power line, when the difference in current between the Line (the black wire) and Neutral (the white wire) is greater than a predetermined level as set by Underwriter""s Laboratories (xe2x80x9cULxe2x80x9d) in UL943. When properly connected, a standard GFCI will protect against any Line-to-Earth Ground faults, thus protecting the end user from potentially fatal electrocution. The standard Class A GFCI was originally intended for use in wall mount applications near any potential ground fault locations such as bathrooms, kitchen counter-tops etc. In order to pass inspection, all outlets are to be electrically tested to ensure proper connection. Since the wiring in the walls does not move, the probability of a loose or broken connection after testing is unlikely. The success of the in-wall GFCI has prompted its use in other alternate locations. One of the prime candidates for a GFCI is its use on the end of an extension cord. The problem with an extension cord is that it is exposed to potential damage, which may break some of the wires in the cord, and thus, render the GFCI inoperative. This is so because the standard Class A GFCI utilizes the power from Line to Neutral in order to operate the unit. Should the Neutral connection be severed, the standard GFCI will no longer function. In order to address this issue, UL now requires, in UL153 for Portable Electric Lamps, that the Class A GFCI incorporate xe2x80x9copen neutral protection.xe2x80x9d This provision protects against only a broken Neutral wire from disabling the GFCI. The xe2x80x9copen neutral provisionxe2x80x9d will not protect against both the Open Neutral and Open Earth Ground failure mode.
Current Open Neutral protection is presently being provided by either an external relay that is powered by the Line-to-Neutral Voltage or by constructing a custom GFCI with an intrinsic relay/circuit breaker combination. Both solutions incorporate another magnetic circuit that tends to either take up more space or impact on product cost.
The major problem with the current standard Class A GFCI is that it requires power from Line to Neutral in order to function. When the Neutral connection is broken, the GFCI no longer has the energy to function. From an ideal electrical point of view, the potential difference between the Neutral and Earth Ground is Zero. In the real world, however, it tends to be on the order of a couple of Volts, depending on Neutral current loss. When the Neutral line is disconnected, it is no longer capable of sinking any current, so if a load were connected, it would match Line potential. Based on this, a Voltage sensitive device/circuit could be placed between Neutral and Earth Ground such that if the potential should exceed a set point, the device would start to conduct current, thus pulling the open Neutral""s potential closer to the potential of Earth Ground. By pulling the open Neutral-to-Earth Ground, the GFCI will then have adequate power to properly function. One such voltage sensitive device that could be utilized is a DAIC, which does not conduct significant current until a threshold Voltage is reached. The Voltage sensitive device could be placed either on the Neutral on the Line input side or the Neutral on the Load Output Side to offer adequate open Neutral protection. Ideally the Voltage sensitive device/circuit should be placed on the Load side since the device could fail, thus triggering the GFCI and therefore offer improved fail-safe protection. Examples of other Voltage sensitive devices/circuits that may be used include, but are not limited to, the following: DIAC, TRIAC, SCR, SIDAC, Bipolar TVS, back-to-back Zener diodes and any other Voltage sensitive devices/circuits.